1. Field of the Invention
The present invention relates to a DC/DC conversion apparatus that includes a LLC full-bridge circuit.
2. Description of the Related Art
In the prior art, a switch power supply is one power supply that utilizes a modern power electronic technology to control a ratio of a turn-on time and a turn-off time of a switch and maintain a stable output voltage, in which a DC/DC conversion apparatus, i.e., direct current-direct current conversion circuit, is a voltage transformer that effectively converts a DC input voltage into a fixed DC output voltage. Generally, the DC/DC conversion apparatus is divided into three types: a boost DC/DC transformer, a buck DC/DC transformer, and a boost-buck DC/DC transformer, and three types of control may be utilized according to requirements. Specifically, by utilizing energy storage characteristics of a capacitor and an inductor, high-frequency switching actions are performed by a controllable switch (MOSFET, etc.), inputted electric energy is stored in the capacitor or the inductor, and the electric energy is released to a load so as to provide energy when the switch is turned off. Its ability to output power or a voltage is related to a duty cycle, i.e., a ratio of a turn-on time of the switch and the whole cycle of the switch.
However, as the power electronic technology is developing rapidly, requirements such as more high-frequency operation, high conversion efficiency, high power density, low noise and other requirements have been proposed to a switch power supply.
FIG. 10 shows an existing DC/DC conversion apparatus 100 that includes a LLC full-bridge circuit. As shown in FIG. 10, the DC/DC conversion apparatus 100 includes a direct-current (DC) voltage source V10, four switch elements Q1˜Q4, an oscillation circuit 20 including an inductor Lr and a capacitor Cr, and a transformation circuit 40 including a transformer 30 and a rectification circuit. In the DC/DC conversion apparatus 100, turn-on and turn-off of individual switch elements Q1˜Q4 are controlled, so as to control energy to be transmitted from a primary side Tr1 of the transformer 30 to its secondary side Tr2.
In the DC/DC conversion apparatus 100 as shown in FIG. 10, a relationship as shown in FIG. 12 exists between a switching frequency fs of the switch elements Q1˜Q4 and a gain G of the DC/DC conversion apparatus 100. As shown in FIG. 12, when the switching frequency fs is equal to a resonance frequency fr of the oscillation circuit 20, the gain of the DC/DC conversion apparatus 100 is at a maximum, and when the switching frequency fs is greater than the resonance frequency fr, the gain of the DC/DC conversion apparatus 100 will decrease as the switching frequency fs increases.
Therefore, when an output terminal of the DC/DC conversion apparatus 100 is a light load, in order to avoid the light load from being damaged due to a large current flowing through the light load, the switching frequency fs of the individual switch elements will usually be increased so as to decrease the gain of the DC/DC conversion apparatus 100, such that a current flowing through the light load decreases to prevent it from being damaged.
However, in the above case, as the operating frequency fs of the switch elements increases, various losses related to the operating frequency fs (e.g., a loss of turning off the MOSFET switch each time), a turn-on loss due to skin effect, proximity effect, and other factors, a loss of a magnetic core, and other losses increase rapidly. For example, within one unit time of 1 s, the times of turning on/off the individual switch elements Q1˜Q4 will increase, which results in an increased power loss on each switch element, such that a conversion efficiency of a power supply voltage decreases.
In view of this, there is a known method of outputting energy nonconsecutively. Specifically, turn-on and turn-off of the individual switch elements Q1˜Q4 are controlled such that energy is transmitted from the primary side Tr1 to the secondary side Tr2 of the transformer 30 nonconsecutively. In other words, as shown in FIG. 11, at time t1-t2, after a current ILLC flowing through the oscillation circuit 20 becomes zero and after a period of time Δt, a direction of a voltage applied to the oscillation circuit 20 is switched, such that without changing the operating frequency fs of the switch elements Q1˜Q4, the gain of the DC/DC conversion apparatus can be reduced to cope with the case of the light load.
However, in the above method, a parasitic inductance Lm on the primary side Tr1 of the transformer 3 connected with the oscillation circuit 20 is larger and a current ILm flowing through the parasitic inductance that cannot be ignored is not considered.
Specifically, FIG. 13 shows a relationship between a current ILr flowing through the inductor Lr and the current ILm flowing through the parasitic inductance Lm, and FIG. 14 shows an output current Iout being obtained based upon the current ILr and the current ILm in FIG. 13. In FIG. 13, a solid line denotes the current ILr flowing through the oscillation circuit 20, in which as the load gets smaller, the current ILr gets smaller, and a dashed line denotes the current ILm flowing through the parasitic inductance Lm, in which the current ILm will not vary with a size of the load. Moreover, as shown in FIG. 13, the current ILr will approach towards the current ILm until finally overlapping along a direction of an arrow A. Corresponding to this, as shown in FIG. 14, when ILr≠ILm, the output current Iout≠0, and when ILr=ILm, the output current Iout=0.
If the direction of the voltage applied to the oscillation circuit 20 is not switched instantly (i.e., to wait for a period of time Δt as shown in FIG. 11) when ILr=ILm, as shown in FIG. 13, although the output current Iout is zero and no energy is transmitted from the primary side Tr1 to the secondary side Tr2 of the transformer at this moment, there is still the current ILm (i.e., ILr) that flows in the oscillation circuit 20 and the capacitor Cr will continue to be charged by the current ILm. Thus, if after the switch elements Q2 and Q4 are turned on, the switch elements Q2 and Q4 continue to be turned on until ILr=ILm such that free oscillation is performed (i.e., to continue for a latency Δt), although no energy is transmitted from the primary side Tr1 to the secondary side Tr2 of the transformer 30 at this moment, there is still the current ILm and a portion of energy is stored in the capacitor Cr. Moreover, when the parasitic inductance Lm on the primary side Tr1 of the transformer 30 is larger such that ILm is larger, this portion of energy will also become larger. In this case, when the switch elements Q2 and Q3 are switched to be turned on and the switch elements Q1 and Q4 are switched to be turned off at time t2, this portion of energy stored in the capacitor Cr will firstly be transmitted to the secondary side Tr2 via the primary side Tr1 of the transformer 30 The result is that a total output energy Eout becomes larger. Although a length of time T of a complete cycle becomes larger due to the addition of a latency Δt, the output energy Pout=Eout/T is related to both Eout and T. Thus, in this case, it cannot be determined whether a total output power Pout will decrease or increase, such that it cannot be determined whether the gain of the DC/DC conversion apparatus 100 will decrease or increase.
Thus, the goal is to decrease the gain in the case of the light load, but it cannot be ensured that the gain will decrease necessarily by the above method for outputting the energy nonconsecutively.
On the other hand, in the DC/DC conversion apparatus that includes a LLC full-bridge circuit, there is also a problem of switching loss (MOSFET, etc.). For the problem of switching loss, a soft-switching technology is usually used in the present technical field.
Soft-switching is in contrast to hard-switching. Generally, resonance is introduced before and after the process of the turn-on and the turn-off, such that a voltage of the switch before it is turned on is firstly reduced to zero and a current of the switch before it is turned off is firstly reduced to zero, which eliminates an overlap of the voltage and the current of the switch during the turn-on and the turn-off and decrease their variation ratios so as to greatly reduce or even eliminate the switching loss. At the same time, the variation ratios of the voltage and the current of the switch during the turn-on and the turn-off are restricted by the resonance process, which also significantly decreases the noise of the switch.
For the process of turning off the switch, an ideal soft turn-off process is such that the current is firstly reduced to zero and then the voltage increases slowly to an off-state value. At this moment, a turn-off loss of the switch is approximately zero. Since the current of the device before it is turned off has been reduced to zero, the problem of inductive turn-off has been solved. This is usually referred to as a zero current switch (ZCS). In addition, for the process of turning on the switch, an ideal soft turn-on process is such that the voltage is firstly reduced to zero and then the current increases slowly to an on-state value. At this moment, a turn-on loss of the switch is approximately zero. Since the voltage of a junction capacitance of the device is also zero, the problem of capacitive turn-on has been solved. This is usually referred to as a zero voltage switch (ZVS).
In the prior art, in order to decrease the loss of the switch when it is turned on or even achieve the zero current switch (ZCS) and/or the zero voltage switch (ZVS), sequences of turning on and off the individual switch elements Q1˜Q4 have to be adjusted appropriately.